UNRPTED VEHICLE TFC ON TXWY FB AT JFK ARPT CAUSES CONCERN FOR FLC COMPLETING A MAX WT TKOF ON RWY 13L.

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I WAS OPERATING THIS FLT AT MAX GROSS WT WITH AN 8 DEG FLAP SETTING; WHICH GREATLY INCREASES THE SPD OF V1. THIS WAS 167 KTS AND VR WAS 173 KTS. DURING THE TKOF ROLL AT APPROX 120 KTS; I NOTICED 2 FIRE TRUCKS APCHING THE RWY DOWNFIELD. AT 150 KTS; THE FO (PF) CALLED OUT THE TRUCKS. I TOLD HIM TO CONTINUE BECAUSE I WAS PRETTY SURE THAT THERE WAS A XING TXWY PAST THE END OF THE RWY THRESHOLD. WE ROTATED AT 173 KTS AND OVERFLEW THE TRUCKS WHO WERE IN FACT ON TXWY FB (SEE ENCLOSED CHART). I FEEL THAT THE TWR SHOULD HAVE NOTIFIED ME OF THE XING TFC. THIS WAS QUITE AN EYE OPENER; BECAUSE I COULD NOT TELL WHERE THESE TRUCKS WERE XING. HAD I NOT BEEN FAMILIAR WITH THIS ARPT; I MAY HAVE ABORTED THE TKOF. I THINK SIGNS SHOULD BE PLACED ON TXWY FB TO TELL TFC TO HOLD SHORT OF THE EXTENSION OF RWY 13L FOR DEPARTING TFC. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID THAT THEY CLRED THE 2 VEHICLES BY AS LITTLE AS 100 FT. HE SAID THAT SINCE THE TRUCKS WERE ON A TXWY AND NOT A SVC ROAD; THEY SHOULD HAVE BEEN UNDER THE CTL OF THE TWR. HE WOULD HAVE EXPECTED AN ADVISORY OR THAT THE VEHICLES WOULD HAVE BEEN REQUIRED TO HOLD SHORT OF THE RWY EXTENSION. HE RECOGNIZED THAT THE TWR PROBABLY EXPECTED THE ACFT TO BE MUCH HIGHER XING THE DEP END. HE WAS ALSO CONCERNED THAT; WHEN QUESTIONED; THE TWR THOUGHT IT WAS 'NO BIG DEAL' AND THAT 'IT HAPPENS OCCASIONALLY.'